Smoke, particularly of the type produced from a burning cigarette, is considered to comprise three phases:                a vapour, or gas phase;        a semi-volatile phase; and        a particulate phase.        
The boiling point of each smoke component largely determines in which phase it exists. For example, components having a low boiling point of less than about 110° C. are considered to be in the gas phase, components having a mid-range boiling point within the range of about 110° C. to 285° C. are considered to be in the semi-volatile phase, and components having a high boiling point of over about 285° C. are considered to be in the particulate phase.
Components forming the gas phase are considered to be fully available for selective filtration. Components in the semi-volatile phase are considered to be partially available for selective filtration. Components in the particulate phase are considered not to be available for selective filtration.
In other words a filter tow adapted to selectively filter tobacco smoke is able to filter substantially all of the components in the gas phase, and a portion of the components in the semi-volatile phase, and substantially none of the components in the particulate phase.
Manufacturers in the Tobacco Industry are seeking to develop means of selective filtration in order to reduce the levels of certain constituents of cigarette smoke, without adversely affecting the desirable taste characteristics associated with the use of cellulose acetate filters. For this purpose, they have devised various constructions of filter rods, involving in many cases the use of porous particles having adsorbent surfaces, particularly activated carbon particles. The inclusion of such particles in a filter rod can have a major impact on the efficiency of the filter, but significant problems are associated with the inclusion of these particles.
One approach has been to have a multi-section filter in which carbon particles are confined to an inner section of the filter, with the part of the filter which, in use, is positioned within the mouth of a user, being a standard cellulose acetate filament filter. In a triple-section filter, for example, the middle section may comprise a bed of loose carbon particles. The use of loose carbon particles can give rise to a manufacturing problem of having to control the unwanted escape of fine particles as dust clouds. In addition, a loose bed of particles in the cigarette filter may be by-passed as a filtration medium due to channeling of the smoke stream passing through it.
Another approach, is to incorporate carbon particles into a filter tow in such a way that they become attached to the surfaces of the filaments.
Early efforts to achieve this concentrated on adhering the carbon particles to the filaments through use of plasticisers or adhesives sprayed onto the tow. U.S. Pat. No. 2,881,770 and U.S. Pat. No. 3,101,723 describe processes of this type and highlight a major problem of deactivation of the carbon particles by the plasticiser or the adhesive.
A more recent attempt to avoid deactivation is described in WO 03/047836. Fine, dry carbon powder is blown onto the filament surfaces of a filter tow. These surfaces have shaped micro-cavities, which are said to hold the powder in place without the need for any deactivating adhesive. Lack of adhesion of the particles can give a greater risk of particle shedding during manufacture and use. Also, the handling of dry powder requires measures to prevent unwanted escape of powder as dust clouds.